1. Field of the Invention
The present invention relates to a zoom lens.
2. Description of the Related Art
In the past, there has been developed various zoom lenses for use in liquid crystal projectors (image projection apparatuses) that use a display element such as a liquid crystal display and project an image formed on the display element onto a screen.
Zoom lenses used in liquid crystal projectors are required to be able to project a large image at a short projection distance and to project a clear image. Thus, the zoom lenses for liquid crystal projectors are required to have a large field angle and high resolution, and particularly not to suffer from significant color drift (shift) or lateral chromatic aberration.
Illumination light source means in such projectors generate heat, which causes an increase in the ambient temperature. So it is demanded that optical characteristics of zoom lenses used in projectors do not change greatly with environmental changes, in particular, the focus state do not change significantly.
As a zoom lens for use in a projector, a negative-lead-type zoom lens in which a negative lens unit is arranged on the enlargement side in order to make the field angle wide (see U.S. Patent Application Publication No. 2003/0234984 and U.S. Pat. No. 6,204,976).
There is a known zoom lens for a projector in which, especially lateral chromatic aberration, among various aberrations, is excellently corrected to reduce color drift (shift) to thereby enhance optical performance (see U.S. Pat. No. 7,016,118).
An effective way of providing high definition projection images easily is to use in the zoom lens an aspherical lens made of a plastic material.
However, optical characteristics of aspherical lenses made of plastic materials change significantly depending on environmental changes. In view of this, in known zoom lens, an aspherical lens made of a glass material is used to provide high definition projection images (see U.S. Pat. No. 6,985,302).
In recent years, it is strongly demanded that zoom lenses used in liquid crystal projectors have a wide field angle, be telecentric on the reduction side and have excellent optical performance throughout the zoom range.
It is also demanded that changes in the optical characteristics, in particular focus changes be small even when environmental changes occur.
To meet such demands, it is desirable that the type of the zoom lens, the configuration of each lens unit, and the material of each lens be designed suitably.